late to the party on this one, but here's my thoughts:
no, it wasn't a deus ex machina at the end, it was a contrived ending which moffat tends to go for(yes, i understand what a deus ex machina is, and some of rtd's ending's aren't that, yet haters still claim they are)
i liked the world they created for this episode. it was like blade runner, tim burton, and final fantasy 7 all combined into one. michael gambon's character was simple, yet tragic, yet simple again.
just like others have said, i didn't like how his controlls stopped working for him the moment he had a heart. this is the downfall of moffat. if he doesn't like something, he just changes it, not explanation. the controlls were isomorphic, not iso-heartic(i'm making that up, yes, i know). the controlls wouldn't just stop working because someone became a better person. just another example of a writer who doesn't believe in continuity.
yes, i liked all the time the doctor spent changing someone's life, however, i have a few problems with it. the doctor, knowing that a spaceship is about to crash, is awfully fun and carefree( even marrying marylin, for a guy who doesn't know how to get along with girls/amy). second, i mean b, i hate how moffat's doctor has matrix/sherlock holmes deduction skills (first demonstrated in the 11th hour, and now when he pointed out the picture behind gambon), but the doctor just overlooks the number on the outside of the girls cry-chamber? really? or when she asked if he was one of her doctor's? that really made me think less of 11. 10 would have asked. ten would have stopped everything he was doing to try to save this girls life, and maybe by doing that, saved gambon's whole life. but 11 was oblivious, and the story had to end bittersweet (another one of moffat's indulgences).
overall it was an ok episode. yes, it was a x-mas episode, just an outerspace/time travel one. i don't see how it was that much different from the x-mas invasion(happened on x-mas), or voyage of the damned. i guess it's still a rtd vs moffat war, and so far, still not sold on moffat's direction of the show.